1. Field of the Invention
The present invention relates to an optical demultiplexer and an optical multiplexer for wavelength division multiplex communication which are used for wavelength division multiplex communication.
2. Description of the Related Background Art
In recent years, the development of optical fiber communication has been remarkable and the number of optical fiber communication lines is rapidly increasing. That is, to say nothing of a public line network, the number of lines for optical fiber communication is increasing also in the field of a computer network that is arranged in a building or a relatively small area. Therefore, a WDM (Wavelength Division Multiplexing) communication system is drawing attention as an effective measure for preventing an increase in the number of optical fiber communication lines. According to the WDM communication system, N light waves of different wavelengths are multiplexed into a single mode optical fiber and are transmitted from the transmission side. On the reception side, the received multiplexed light is demultiplexed to the N light waves of the different wavelengths. Thus, an amount of information which can be transmitted in the optical fiber can be increased to a value that is N times as large as the amount of information which can be transmitted in the case of transmitting only a light signal of a single wavelength.
In the WDM communication system, an optical demultiplexer and an optical multiplexer for wavelength division multiplex communication are provided for demultiplexing to or multiplexing the N light waves of the different wavelengths. In such an optical demultiplexer and optical multiplexer for wavelength division multiplex communication, excellent demultiplexing and multiplexing characteristics are naturally demanded and the optical demultiplexer and the optical multiplex are also required to be obtained at a low price.
An example of a conventional optical demultiplexer for wavelength division multiplex communication will now be described with reference to FIG. 4.
FIG. 4 is a constructional diagram of the conventional optical demultiplexer for wavelength division multiplex communication.
In the conventional demultiplexer, as described in detail in Japanese Patent Application No. 2000-366746 (by the same applicant as that of the present invention), a light multiplexed signal comprising four light waves of wavelengths λ1 to λ4 is emitted as a spherical wave 22 from an optical fiber 21-1. The spherical wave 22 is collimated by a collimator lens 23-1 of an axial offset type and becomes a parallel wave 24. The light wave of the wavelength λ1 in the parallel wave 24 is reflected by a WDM (Wavelength Division Multiplexing) filter (λ1) 25-1, is converged by a collimator lens 23-2 of an axial offset type and, further, enters an optical fiber 21-2.
When the parallel wave 24 including the three light waves of the wavelengths λ2 to λ4 passes through the WDM filter (λ1) 25-1, the light wave of the wavelength λ2 in the parallel wave 24 is reflected by a WDM filter (λ2) 25-2, is converged by a collimator lens 23-3 of an axial offset type and, further, enters an optical fiber 21-3. In a manner similar to the above, the light waves of the wavelengths λ3 and λ4 also enter optical fibers 21-4 and 21-5 after being reflected by a WDM filter (λ3) 25-3 and a mirror 4-1 and converged by a collimator lens 23-4 and 23-5, respectively.
In such an optical demultiplexer for wavelength division multiplex communication, the whole optical system can be constructed in a compact size by using a lens array of a pitch of 250 μm that is compatible with an MT connector. Upon manufacturing, alignment is only necessary between the lens array and a fiber array is necessary, and alignment of other portions is unnecessary. Further, there are advantages in installation such that the demultiplexer can be manufactured by simply laminating optical substrates, and the like.
The conventional demultiplexer as mentioned above has the following problem to be solved. That is, since the WDM filter slightly reflects and transmits light waves having wavelengths other than the wavelength set in the WDM filter, there is a problem to be solved such that crosstalks among the light waves of different wavelengths increase.